Systems and methods for electrosurgical tissue treatment

ABSTRACT

Systems and methods are provided for removal (ablation) or modification of soft tissue with high frequency voltage through a capacitive charge process. Specifically, one of the conductors of a capacitor device maybe positioned adjacent to, or in the region of, a body structure. The other conductor is spaced, and electrically insulated from, the first conductor. A high frequency voltage is applied across the conductors to create a potential difference that results in a charge on the conductors and creates an electric field therebetween. High frequency alternating current flows between the conductors to a degree dependent on the capacitance and the current frequency. The charge on the first conductor is sufficient to modify the adjacent tissue. In some variations the charge is sufficient to heat the tissue for tissue coagulation or tissue shrinkage (i.e., collagen contraction). In other variations, the charge is sufficient to ablate or volumetrically remove the tissue.

RELATED APPLICATIONS

The present application is a non-provisional application of U.S. Provisional Patent application No. 60/151,703, filed Aug. 31, 1999. This application is also a continuation-in-part of U.S. patent application Ser. No. 09/248,763, filed Feb. 12, 1999 now U.S. Pat. No. 6,149,620, which is a continuation-in-part of U.S. patent application Ser. No. 08/795,686, filed Feb. 5, 1997 now U.S. Pat. No. 5,871,469, which is a continuation of Ser. No. 08/561,958 filed Nov. 22, 1995, now U.S. Pat. No. 5,697,882, filed Nov. 22, 1995, the complete disclosures of which are incorporated herein by reference for all purposes.

The present invention is related to commonly assigned co-pending U.S. patent application Ser. No. 09/177,861 filed Oct. 23, 1998, and application Ser. No. 08/977,845, filed Nov. 25, 1997, which is a continuation-in-part of application Ser. No. 08/562,332, filed Nov. 22, 1995, and U.S. patent application Ser. No. 09/010,382, filed Jan. 21, 1998, the complete disclosure of which is incorporated herein by reference. The present invention is also related to commonly assigned co-pending U.S. patent application Ser. No. 09/162,117, filed Sept. 28, 1998, and U.S. patent Ser. No. 08/990,374, filed Dec. 15, 1997, which is a continuation-in-part of U.S. patent application Ser. No. 08/485,219, filed on Jun. 7, 1995, now U.S. Pat. No. 5,697,281, patent application Ser. Nos. 09/109,219, 09/058,571, 08/874,173 and 09/002,315, filed on Jun. 30, 1998, Apr. 10, 1998, Jun. 13, 1997, and Jan. 2, 1998, respectively and U.S. patent application Ser. No. 09/054,323, filed on Apr. 2, 1998, and U.S. patent application Ser. No. 09/032,375, filed Feb. 27, 1998, Ser. No. 08/942,580, filed on Oct. 2, 1997, U.S. application Ser. No. 08/753,227, filed on Nov. 22, 1996, U.S. application Ser. No. 08/687792, filed on Jul. 18, 1996, the complete disclosures of which are incorporated herein by reference for all purposes.

BACKGROUND OF THE INVENTION

The present invention relates generally to the field of electrosurgery and, more particularly, to surgical devices and methods which employ high frequency voltage to cut and ablate body tissue.

Conventional electrosurgical methods are widely used since they generally reduce patient bleeding associated with tissue cutting operations and improve the surgeon's visibility. These traditional electrosurgical techniques for treatment have typically relied on thermal methods to rapidly heat and vaporize liquid within tissue and to cause cellular destruction. In conventional monopolar electrosurgery, for example, electric current is directed along a defined path from the exposed or active electrode through the patient's body to the return electrode, which is externally attached to a suitable location on the patient's skin. In addition, since the defined path through the patient's body has a relatively high electrical impedance, large voltage differences must typically be applied between the active and return electrodes to generate a current suitable for cutting or coagulation of the target tissue. This current, however, may inadvertently flow along localized pathways in the body having less impedance than the defined electrical path. This situation will substantially increase the current flowing through these paths, possibly causing damage to or destroying tissue along and surrounding this pathway.

Bipolar electrosurgical devices have an inherent advantage over monopolar devices because the return current path does not flow through the patient beyond the immediate site of application of the bipolar electrodes. In bipolar devices, both the active and return electrode are typically exposed so that they may both contact tissue, thereby providing a return current path from the active to the return electrode through the tissue. One drawback with this configuration, however, is that the return electrode may cause tissue desiccation or destruction at its contact point with the patient's tissue.

Another limitation of conventional bipolar and monopolar electrosurgery devices is that they are not suitable for the precise removal (i.e., ablation) or tissue. For example, conventional electrosurgical cutting devices typically operate by creating a voltage difference between the active electrode and the target tissue, causing an electrical arc to form across the physical gap between the electrode and tissue. At the point of contact of the electric arcs with tissue, rapid tissue heating occurs due to high current density between the electrode and tissue. This high current density causes cellular fluids to rapidly vaporize into steam, thereby producing a “cutting effect” along the pathway of localized tissue heating. The tissue is parted along the pathway of evaporated cellular fluid, inducing undesirable collateral tissue damage in regions surrounding the target tissue site.

The use of electrosurgical procedures (both monopolar and bipolar) in electrically conductive environments can be further problematic. For example, many arthroscopic procedures require flushing of the region to be treated with isotonic saline, both to maintain an isotonic environment and to keep the field of view clear. However, the presence of saline, which is a highly conductive electrolyte, can cause shorting of the active electrode(s) in conventional monopolar and bipolar electrosurgery. Such shorting causes unnecessary heating in the treatment environment and can further cause non-specific tissue destruction.

Present electrosurgical techniques used for tissue ablation also suffer from an inability to control the depth of necrosis in the tissue being treated. Most electrosurgical devices rely on creation of an electric arc between the treating electrode and the tissue being cut or ablated to cause the desired localized heating. Such arcs, however, often create very high temperatures causing a depth of necrosis greater than 500 μm, frequently greater than 800 μm, and sometimes as great as 1700 μm. The inability to control such depth of necrosis is a significant disadvantage in using electrosurgical techniques for tissue ablation, particularly in arthroscopic procedures for ablating and/or reshaping fibrocartilage, articular cartilage, meniscal tissue, and the like.

In an effort to overcome at least some of these limitations of electrosurgery, laser apparatus have been developed for use in arthroscopic and other surgical procedures. Lasers do not suffer from electrical shorting in conductive environments, and certain types of lasers allow for very controlled cutting with limited depth of necrosis. Despite these advantages, laser devices suffer from their own set of deficiencies. In the first place, laser equipment can be very expensive because of the costs associated with the laser light sources. Moreover, those lasers which permit acceptable depths of necrosis (such as excimer lasers, erbium:YAG lasers, and the like) provide a very low volumetric ablation rate, which is a particular disadvantage in cutting and ablation of fibrocartilage, articular cartilage, and meniscal tissue. The holmium:YAG and Nd:YAG lasers provide much higher volumetric ablation rates, but are much less able to control depth of necrosis than are the slower laser devices. The CO₂ lasers provide high rate of ablation and low depth of tissue necrosis, but cannot operate in a liquid-filled cavity.

Excimer lasers, which operate in an ultraviolet wavelength, cause photodissociation of human tissue, commonly referred to as cold ablation. Through this mechanism, organic molecules can be disintegrated into light hydrocarbon gases that are removed from the target site. Such photodissociation reduces the likelihood of thermal damage to tissue outside of the target site. Although promising, excimer lasers must be operated in pulses so that ablation plumes created during operation can clear. This prevents excessive secondary heating of the plume of ablation products which can increase the likelihood of collateral tissue damage as well as a decrease in the rate of ablation. Unfortunately, the pulsed mode of operation reduces the volumetric ablation rate, which may increase the time spent in surgery.

SUMMARY OF THE INVENTION

The present invention provides systems, apparatus and methods for selectively applying electrical energy to body tissue.

In one embodiment, the method of the present invention comprises positioning an electrosurgical probe or catheter adjacent the target site so one or more active electrode(s) and one or more return electrode(s) are positioned in the region of the body structure. The return electrode(s) are electrically insulated from the active electrode(s) and the patient's body, and a high frequency voltage difference is applied between the active and return electrode(s) to modify or ablate at least a portion of the body structure.

In a specific configuration, the active and return electrode(s) are spaced from each other on the distal end portion of a surgical instrument. The return electrode(s) are insulated from the active electrode(s) and the patient's body by an insulator, preferably a thin, insulating jacket that surrounds the shaft of the instrument and the return electrode. In one embodiment, the active electrode(s) are immersed in electrically conductive fluid such that a conductive path is created between the insulator and the active electrode(s). The electrically conductive fluid may be delivered directly to the active electrode(s) or the entire target site may be submersed within the conductive fluid. Applicant believes that the conductive fluid creates a virtual conductor or virtual electrode that includes the active electrode(s) and the conductive fluid surrounding the distal end portion of the instrument. With this configuration, a capacitor is created with the return electrode functioning as the second parallel plate or conductor and the insulator functioning as the dielectric between the conductors. When high frequency voltage is applied between the conductors, a potential difference is created that results in a charge on the conductors, creating an electric field therebetween.

According to the present invention, the active and return electrode(s) are configured such that the charge on the active electrode(s) is sufficient to modify tissue in contact with, or in close proximity to, the active electrode(s). In some embodiments, this charge is sufficient to ablate or volumetrically remove the tissue. Since the current is not flowing into the tissue, this tissue modification or ablation is accomplished at substantially lower temperatures than traditional electrosurgery, which reduces collateral tissue damage. In addition, the current does not penetrate beyond the target tissue site, which further reduces damage to the surrounding and underlying tissue. Moreover, the electric current is precisely controlled, which allows the device to be used adjacent to electrically sensitive structures, such as nerves, the heart or the spine. Another advantage of the present invention is that the alternating current flow between the conductors is more uniform across the surface of the return electrode, rather than collecting at the distal corner of the return electrode as may occur with traditional electrosurgery devices.

In a specific embodiment, a sufficient high frequency voltage is applied between the active and return electrodes to generate a plasma adjacent to the active electrode(s), and to volumetrically remove or ablate at least a portion of the target tissue. The high frequency voltage generates electric fields around the active electrode(s) with sufficient energy to ionize the conductive fluid adjacent to the active electrode(s). Within the ionized gas or plasma, free electrons are accelerated, and electron-atoms collisions liberate more electrons, and the process cascades until the plasma contains sufficient energy to break apart the tissue molecules, causing molecular dissociation and ablation of the target tissue.

In some embodiments, the high frequency voltage applied to the electrode terminal(s) is sufficient to vaporize the electrically conductive fluid (e.g., gel or saline) between the electrode terminal(s) and the tissue. Within the vaporized fluid, an ionized plasma is formed and charged particles (e.g., electrons) are accelerated towards the tissue to cause the molecular breakdown or disintegration of several cell layers of the tissue. This molecular dissociation is accompanied by the volumetric removal of the tissue. The short range of the accelerated charged particles within the plasma layer confines the molecular dissociation process to the surface layer to minimize damage and necrosis to the underlying tissue. This process can be precisely controlled to effect the volumetric removal of tissue as thin as 10 to 150 microns with minimal heating of, or damage to, surrounding or underlying tissue structures. A more complete description of this phenomena is described in commonly assigned U.S. Pat. No. 5,697,882.

In one application, the present invention is particularly useful for selectively contracting soft collagen tissue and other body structures, while limiting thermal damage or molecular dissociation of such tissue and limiting the thermal damage to tissue adjacent to and underlying the treatment site. The systems and methods of the present invention are particularly useful for surgical procedures in electrically conducting environments, such as arthroscopic procedures in the joints, e.g., shoulder, knee, hip, hand, foot, elbow or the like. A more complete description of bipolar methods for shrinking tissue in joints can be found in U.S. patent Ser. No. 09/273,612, the complete disclosure of which is hereby incorporated herein by reference. In this application, the return electrode is provided on the perimeter of the shaft such that the conductive fluid in the joint provides a conductive path between the active and return electrodes. One potential problem with this configuration is that the return electrode may cause burning of tissue near the portal within the joint due to space constraints within the joint, particularly the shoulder capsule. According to the present invention, the return electrode is insulated from the conductive fluid and from the patient's body to eliminate this potential burning of the tissue. A sufficient high frequency voltage difference can be applied to the active and return electrodes to effect contraction of collagen tissue in the joint without causing molecular dissociation or charring of the tissue.

In another application, the present invention is particularly useful for reducing or eliminating the effects of restenosis in coronary arteries by selectively removing tissue ingrowth in or around stents anchored therein. In this method, an electrosurgical catheter is advanced within the body passage such that an electrode terminal and a return electrode are positioned near the occlusive media. High frequency voltage is applied between the active and return electrodes as described above to produce a charge sufficient to volumetrically remove the occlusive media in situ. In exemplary embodiments, the high frequency voltage is sufficient to effect molecular dissociation or disintegration of the occlusive media, thus converting the solid media into non-condensable gases. According to the present invention, the return electrode is insulated from the active electrode and the patient's body to eliminate the potential for contact between the return electrode and the body lumen, which could otherwise could thermal damage to the walls of the vessel.

The present invention is particularly useful in a lumen containing a lumenal prosthesis, such as a stent, stent-graft or graft, which may be metallic, non-metallic or a non-metallic coated metallic structure. Restenosis often occurs when arthermateous media or thrombus moves or grows through or around the cylindrical wall of the prosthesis to partially occlude the body passage. In this application, it is particularly useful to insulate the return electrode from the conductive stent, and to control the flow of current at the target site to minimize current flow through the stent.

Apparatus according to the present invention generally include an electrosurgical instrument having a shaft with proximal and distal ends, one or more active electrode(s) at the distal end and one or more connectors coupling the active electrode(s) to a source of high frequency electrical energy. The apparatus further includes one or more return electrode(s) at or near the distal end of the instrument shaft and being electrically insulated from the active electrode(s) and the patient's body. In some embodiments, the instrument will comprise a catheter designed for percutaneous and/or transluminal delivery. In other embodiments, the instrument will comprise a more rigid probe designed for percutaneous or direct delivery in either open procedures or port access type procedures. In both embodiments, the apparatus will include a high frequency power supply for applying a high frequency voltage to the electrode terminal(s).

In a specific configuration, the instrument comprises one or more active electrode(s) at the distal end, and a return electrode on the instrument shaft and spaced proximally from the active electrode(s). A thin, electrically insulating jacket surrounds the return electrode to insulate the return electrode from the active electrode. The insulating jacket has a material and thickness selected that will allow for sufficient charge to build on the electrodes to ablate or otherwise modify tissue adjacent to the active electrode(s). In the representative embodiment, the insulator comprises a polytetrafluoroethylene, polyimide, teflon, urethane or silicone material, and has a thickness in the range of about 0.01 to 0.5 mm.

In preferred embodiments, the apparatus will further include a supply of electrically conductive fluid and a fluid delivery element for delivering electrically conducting fluid to the electrode terminal(s) and the target site. The fluid delivery element may be located on the instrument, e.g., a fluid lumen or tube, or it may be part of a separate instrument. Alternatively, an electrically conducting gel or spray, such as a saline electrolyte or other conductive gel, may be applied to the target site. In this embodiment, the apparatus may not have a fluid delivery element. In both embodiments, the electrically conducting fluid will preferably generate a current flow path between the active electrode(s) and the portion of the insulator immediately surrounding the return electrode.

The electrosurgical instrument will preferably include an electrically insulating electrode support member, preferably an inorganic support material (e.g., ceramic, glass, glass/ceramic, silicone etc.) having a tissue treatment surface at the distal end of the instrument shaft. One or more electrode terminal(s) are coupled to, or integral with, the electrode support member such that the electrode terminal(s) are spaced from the return electrode. In one embodiment, the instrument includes an electrode array having a plurality of electrically isolated electrode terminals embedded into the electrode support member such that the electrode terminals extend about 0.0 mm to about 10 mm distally from the tissue treatment surface of the electrode support member. In some embodiments, the probe will further include one or more lumens for delivering electrically conductive fluid and/or aspirating the target site to one or more openings around the tissue treatment surface of the electrode support member. In an exemplary embodiment, the lumen will extend through a fluid tube exterior to the probe shaft that ends proximal to the return electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an electrosurgical system incorporating a power supply and an electrosurgical probe for treating articular cartilage according to the present invention;

FIG. 2 schematically illustrates one embodiment of a power supply according to the present invention;

FIG. 3 is a side view of an electrosurgical probe according to the present invention;

FIG. 4 is an enlarged detailed cross-sectional view of the working end of the electrosurgical probe of FIG. 3;

FIG. 5 is a perspective view of another embodiment of the present invention incorporating an aspiration lumen;

FIG. 6 is a perspective view of another embodiment of the present invention incorporating an aspiration lumen and a fluid delivery lumen;

FIG. 7 is a perspective view of an electrosurgical catheter system according to the present invention; and

FIG. 8 is a detailed cross-sectional view of the working end of the catheter of FIG. 7.

DESCRIPTION OF SPECIFIC EMBODIMENTS

In the present invention, high frequency (RF) electrical energy is applied to one or more electrode terminals to remove and/or modify body tissue. The techniques of the present invention may be performed in a conventional open surgery environment or in a minimally invasive manner using cannulas or port access devices. The systems and methods of the present invention can be applied to a number of medical procedures including open procedures, intravascular procedures, interventional cardiology procedures, urology, laparascopy, arthroscopy, thoracoscopy or other cardiac procedures, cosmetic surgery, orthopedics, gynecology, otorhinolaryngology, spinal and neurologic procedures, oncology and the like.

The present invention is particularly useful in procedures where the tissue site is flooded or submerged with an electrically conducting fluid, such as arthroscopic surgery of the knee, shoulder, ankle, hip, elbow, hand or foot. Specifically, the present invention is useful in the resection and/or ablation of the meniscus and the synovial tissue within a joint during an arthroscopic procedure. In addition, tissues which may be treated by the system and method of the present invention include, but are not limited to, prostate tissue and leiomyomas (fibroids) located within the uterus, gingival tissues and mucosal tissues located in the mouth, tumors, scar tissue, myocardial tissue, collagenous tissue within the eye or epidermal and dermal tissues on the surface of the skin. The present invention is also useful for resecting tissue within accessible sites of the body that are suitable for electrode loop resection, such as the resection of prostate tissue, leiomyomas (fibroids) located within the uterus and other diseased tissue within the body.

The present invention is particularly useful for treating tissue in the head and neck, such as the ear, mouth, pharynx, larynx, esophagus, nasal cavity and sinuses. The head and neck procedures may be performed through the mouth or nose using speculae or gags, or using endoscopic techniques, such as functional endoscopic sinus surgery (FESS). These procedures may include the removal of swollen tissue, chronically-diseased inflamed and hypertrophic mucus linings, polyps, turbinates and/or neoplasms from the various anatomical sinuses of the skull, the turbinates and nasal passages, in the tonsil, adenoid, epi-glottic and supra-glottic regions, and salivary glands, submucus resection of the nasal septum, excision of diseased tissue and the like. In other procedures, the present invention may be useful for collagen shrinkage, ablation and/or hemostasis in procedures for treating swollen tissue (e.g., turbinates) or snoring and obstructive sleep apnea (e.g., soft palate, such as the uvula, or tongue/pharynx stiffening, and midline glossectomies), for gross tissue removal, such as tonsillectomies, adenoidectomies, tracheal stenosis and vocal cord polyps and lesions, or for the resection or ablation of facial tumors or tumors within the mouth and pharynx, such as glossectomies, laryngectomies, acoustic neuroma procedures and nasal ablation procedures. In addition, the present invention is useful for procedures within the ear, such as stapedotomies, tympanostomies or the like.

The present invention may also be useful for treating tissue or other body structures in the brain or spine. These procedures include tumor removal, laminectomy/disketomy procedures for treating herniated disks, decompressive laminectomy for stenosis in the lumbosacral and cervical spine, medial facetectomy, posterior lumbosacral and cervical spine fusions, treatment of scoliosis associated with vertebral disease, foraminotomies to remove the roof of the intervertebral foramina to relieve nerve root compression and anterior cervical and lumbar diskectomies. These procedures may be performed through open procedures, or using minimally invasive techniques, such as thoracoscopy, arthroscopy, laparascopy or the like.

The present invention may also be useful for cosmetic and plastic surgery procedures in the head and neck. For example, the present invention is particularly useful for ablation and sculpting of cartilage tissue, such as the cartilage within the nose that is sculpted during rhinoplasty procedures. The present invention may also be employed for skin tissue removal and/or collagen shrinkage in the epidermis or dermis tissue in the head and neck, e.g., the removal of pigmentations, vascular lesions (e.g., leg veins), scars, tattoos, etc., and for other surgical procedures on the skin, such as tissue rejuvenation, cosmetic eye procedures (blepharoplasties), wrinkle removal, tightening muscles for facelifts or browlifts, hair removal and/or transplant procedures, etc.

The invention may also be useful for maintaining patency in body passages subject to occlusion by invasive tissue growth. The apparatus and methods of the invention may be used to open and maintain patency in virtually any hollow body passage which may be subject to occlusion by invasive cellular growth or invasive solid tumor growth. Suitable hollow body passages include ducts, orifices, lumens, and the like, with exemplary body passages including the coronary arteries. The present invention is particularly useful for reducing or eliminating the effects of restenosis in coronary arteries by selectively removing tissue ingrowth in or around stents anchored therein.

According to the invention, soft tissue is modified or removed (i.e., ablated) with high frequency voltage through a capacitive charge process. Specifically, one of the conductors of a capacitor device is positioned adjacent to, or in the region of, a body structure. The other conductor is spaced, and electrically insulated from, the first conductor. A high frequency voltage is applied across the conductors to create a potential difference that results in a charge on the conductors and creates an electric field therebetween. High frequency alternating current flows between the conductors to a degree dependent on the capacitance and the current frequency. The charge on the first conductor is sufficient to modify the adjacent tissue. In some embodiments, the charge is sufficient to heat the tissue for tissue coagulation or tissue shrinkage (i.e., collagen contraction). In other embodiments, the charge is sufficient to ablate or volumetrically remove the tissue.

The amount of charge, Q, built up on the conductors of a capacitor is proportional to the applied voltage. The constant of proportionality, C, is the capacitance in farads (F) and depends on the capacitor construction. The capacitance, C, is generally proportional to the dielectric constant of the insulator times the surface area of the conductors times the distance between the two conductors (i.e., the thickness of the insulator). In one aspect of the present invention, a conductive medium is present between the first or active conductor and the insulator which effectively creates a virtual conductor that includes the first conductor and the conductive medium. In this manner, the conductors may be spaced a larger distance away from each other (typically from 1 to 50 mm) without effectively reducing the capacitance of the capacitor because the distance between the conductors is measured as the distance from the conductive fluid/insulator interface and the second conductor/insulator interface.

In one aspect of the invention, the first conductor of the capacitor comprises one or more active electrodes positioned at the distal end of an instrument shaft. The second conductor comprises a return electrode spaced proximally from the active electrode(s) on the shaft, and the insulator comprises a thin, insulating jacket around the return electrode. In this manner, the return electrode is also insulated from the patient's body. Conductive fluid is present around the instrument shaft such that the insulator and the active electrode(s) contact the conductive fluid. The insulator comprises a teflon, silicone, urethane, polyimide or similar material and will typically have a thickness of about 0.01 to 0.5 mm which effectively represents the distance between the two conductors of the capacitor. The return electrode will typically have a larger surface area than the active electrode(s) to provide a more uniform distribution of charge across the return electrode surface and to minimize the charge at any point on this surface. The active electrodes, on the other hand, will typically have a smaller surface area to maximize charge on the active electrode(s)′ surface(s).

In one embodiment, the body tissue is volumetrically removed or ablated. In this procedure, the high frequency voltage difference applied across the conductors is sufficient to develop high electric field intensities in the vicinity of the target tissue. The high electric field intensities adjacent the first conductor or active electrode(s) lead to electric field induced molecular breakdown of target tissue through molecular dissociation (rather than thermal evaporation or carbonization). Applicant believes that the tissue structure is volumetrically removed through molecular disintegration of larger organic molecules into smaller molecules and/or atoms, such as hydrogen, oxygen, oxides of carbon, hydrocarbons and nitrogen compounds. This molecular disintegration completely removes the tissue structure, as opposed to dehydrating the tissue material by the removal of liquid within the cells of the tissue, as is typically the case with electrosurgical desiccation and vaporization.

The high electric field intensities may be generated by applying a high frequency voltage that is sufficient to vaporize an electrically conducting fluid over at least a portion of the active electrode(s) in the region between the distal tip of the electrode terminal(s) and the target tissue. The electrically conductive fluid may be a liquid or gas, such as isotonic saline or blood, delivered to the target site, or a viscous fluid, such as a gel, applied to the target site. Since the vapor layer or vaporized region has a relatively high electrical impedance, it increases the voltage differential between the electrode terminal tip and the tissue and causes ionization within the vapor layer due to the presence of an ionizable species (e.g., sodium when isotonic saline is the electrically conducting fluid). This ionization, under the conditions described herein, induces the discharge of energetic electrons and photons from the vapor layer and to the surface of the target tissue. This energy may be in the form of energetic photons (e.g., ultraviolet radiation), energetic particles (e.g., electrons or ions) or a combination thereof. A more detailed description of this phenomena, termed Coblation® can be found in commonly assigned U.S. Pat. No. 5,697,882 the complete disclosure of which is incorporated herein by reference.

Applicant believes that the principle mechanism of tissue removal in the Coblation® mechanism of the present invention is energetic electrons or ions that have been energized in a plasma adjacent to the active electrode(s). When a liquid is heated enough that atoms vaporize off the surface faster than they recondense, a gas is formed. When the gas is heated enough that the atoms collide with each other and knock their electrons off in the process, an ionized gas or plasma is formed (the so-called “fourth state of matter”). A more complete description of plasma can be found in Plasma Physics, by R. J. Goldston and P. H. Rutherford of the Plasma Physics Laboratory of Princeton University (1995). When the density of the vapor layer (or within a bubble formed in the electrically conducting liquid) becomes sufficiently low (i.e., less than approximately 1020 atoms/cm3 for aqueous solutions), the electron mean free path increases to enable subsequently injected electrons to cause impact ionization within these regions of low density (i.e., vapor layers or bubbles). Once the ionic particles in the plasma layer have sufficient energy, they accelerate towards the target tissue. Energy evolved by the energetic electrons (e.g., 3.5 eV to 5 eV) can subsequently bombard a molecule and break its bonds, dissociating a molecule into free radicals, which then combine into final gaseous or liquid species.

Plasmas may be formed by heating a small of gas and ionizing the gas by driving an electric current through it, or by shining radio waves into the gas. Generally, these methods of plasma formation give energy to free electrons in the plasma directly, and then electron-atom collisions liberate more electrons, and the process cascades until the desired degree of ionization is achieved. Often, the electrons carry the electrical current or absorb the radio waves and, therefore, are hotter than the ions. Thus, in applicant's invention, the electrons, which are carried away from the tissue towards the return electrode, carry most of the plasma's heat with them, allowing the ions to break apart the tissue molecules in a substantially non-thermal manner.

In some embodiments, the present invention applies high frequency (RF) electrical energy in an electrically conducting fluid environment to remove (i.e., resect, cut or ablate) a tissue structure and to seal transected vessels within the region of the target tissue. The present invention is particularly useful for sealing larger arterial vessels, e.g., on the order of 1 mm or greater. In some embodiments, a high frequency power supply is provided having an ablation mode, wherein a first voltage is applied to an electrode terminal sufficient to effect molecular dissociation or disintegration of the tissue, and a coagulation mode, wherein a second, lower voltage is applied to an electrode terminal (either the same or a different electrode) sufficient to achieve hemostasis of severed vessels within the tissue. In other embodiments, an electrosurgical instrument is provided having one or more coagulation electrode(s) configured for sealing a severed vessel, such as an arterial vessel, and one or more electrode terminals configured for either contracting the collagen fibers within the tissue or removing (ablating) the tissue, e.g., by applying sufficient energy to the tissue to effect molecular dissociation. In the latter embodiments, the coagulation electrode(s) may be configured such that a single voltage can be applied to coagulate with the coagulation electrode(s), and to ablate with the electrode terminal(s). In other embodiments, the power supply is combined with the coagulation instrument such that the coagulation electrode is used when the power supply is in the coagulation mode (low voltage), and the electrode terminal(s) are used when the power supply is in the ablation mode (higher voltage).

In one method of the present invention, one or more electrode terminals are brought into close proximity to tissue at a target site, and the power supply is activated in the ablation mode such that sufficient voltage is applied between the electrode terminals and the return electrode to volumetrically remove the tissue through molecular dissociation, as described below. During this process, vessels within the tissue will be severed. Smaller vessels will be automatically sealed with the system and method of the present invention. Larger vessels, and those with a higher flow rate, such as arterial vessels, may not be automatically sealed in the ablation mode. In these cases, the severed vessels may be sealed by activating a control (e.g., a foot pedal) to reduce the voltage of the power supply into the coagulation mode. In this mode, the electrode terminals may be pressed against the severed vessel to provide sealing and/or coagulation of the vessel. Alternatively, a coagulation electrode located on the same or a different instrument may be pressed against the severed vessel. Once the vessel is adequately sealed, the surgeon activates a control (e.g., another foot pedal) to increase the voltage of the power supply back into the ablation mode.

The present invention is also useful for removing or ablating tissue around nerves, such as spinal, or cranial nerves, e.g., optic nerve, facial nerves, vestibulocochlear nerves and the like. One of the significant drawbacks with the prior art microdebriders and lasers is that these devices do not differentiate between the target tissue and the surrounding nerves or bone. Therefore, the surgeon must be extremely careful during these procedures to avoid damage to the bone or nerves within and around the nasal cavity. In the present invention, the Coblation® process for removing tissue results in extremely small depths of collateral tissue damage as discussed above. This allows the surgeon to remove tissue close to a nerve without causing collateral damage to the nerve fibers.

In addition to the generally precise nature of the novel mechanisms of the present invention, applicant has discovered an additional method of ensuring that adjacent nerves are not damaged during tissue removal. According to the present invention, systems and methods are provided for distinguishing between the fatty tissue immediately surrounding nerve fibers and the normal tissue that is to be removed during the procedure. Nerves usually comprise a connective tissue sheath, or epineurium, enclosing the bundles of nerve fibers, each bundle being surrounded by its own sheath of connective tissue (the perineurium) to protect these nerve fibers. The outer protective tissue sheath or epineurium typically comprises a fatty tissue (e.g., adipose tissue) having substantially different electrical properties than the normal target tissue, such as the turbinates, polyps, mucus tissue or the like, that are, for example, removed from the nose during sinus procedures. The system of the present invention measures the electrical properties of the tissue at the tip of the probe with one or more electrode terminal(s). These electrical properties may include electrical conductivity at one, several or a range of frequencies (e.g., in the range from 1 kHz to 100 MHz), dielectric constant, capacitance or combinations of these. In this embodiment, an audible signal may be produced when the sensing electrode(s) at the tip of the probe detects the fatty tissue surrounding a nerve, or direct feedback control can be provided to only supply power to the electrode terminal(s) either individually or to the complete array of electrodes, if and when the tissue encountered at the tip or working end of the probe is normal tissue based on the measured electrical properties.

In one embodiment, the current limiting elements (discussed in detail above) are configured such that the electrode terminals will shut down or turn off when the electrical impedance reaches a threshold level. When this threshold level is set to the impedance of the fatty tissue surrounding nerves, the electrode terminals will shut off whenever they come in contact with, or in close proximity to, nerves. Meanwhile, the other electrode terminals, which are in contact with or in close proximity to nasal tissue, will continue to conduct electric current to the return electrode. This selective ablation or removal of lower impedance tissue in combination with the Coblation® mechanism of the present invention allows the surgeon to precisely remove tissue around nerves or bone. Applicant has found that the present invention is capable of volumetrically removing tissue closely adjacent to nerves without impairment the function of the nerves, and without significantly damaging the tissue of the epineurium. One of the significant drawbacks with the prior art microdebriders and lasers is that these devices do not differentiate between the target tissue and the surrounding nerves or bone. Therefore, the surgeon must be extremely careful during these procedures to avoid damage to the bone or nerves within and around the nasal cavity. In the present invention, the Coblation® process for removing tissue results in extremely small depths of collateral tissue damage as discussed above. This allows the surgeon to remove tissue close to a nerve without causing collateral damage to the nerve fibers.

In addition to the above, applicant has discovered that the Coblation® mechanism of the present invention can be manipulated to ablate or remove certain tissue structures, while having little effect on other tissue structures. As discussed above, the present invention uses a technique of vaporizing electrically conductive fluid to form a plasma layer or pocket around the electrode terminal(s), and then inducing the discharge of energy from this plasma or vapor layer to break the molecular bonds of the tissue structure. Based on initial experiments, applicants believe that the free electrons within the ionized vapor layer are accelerated in the high electric fields near the electrode tip(s). When the density of the vapor layer (or within a bubble formed in the electrically conducting liquid) becomes sufficiently low (i.e., less than approximately 1020 atoms/cm3 for aqueous solutions), the electron mean free path increases to enable subsequently injected electrons to cause impact ionization within these regions of low density (i.e., vapor layers or bubbles). Energy evolved by the energetic electrons (e.g., 4 to 5 eV) can subsequently bombard a molecule and break its bonds, dissociating a molecule into free radicals, which then combine into final gaseous or liquid species.

The energy evolved by the energetic electrons may be varied by adjusting a variety of factors, such as: the number of electrode terminals; electrode size and spacing; electrode surface area; asperities and sharp edges on the electrode surfaces; electrode materials; applied voltage and power; current limiting means, such as inductors; electrical conductivity of the fluid in contact with the electrodes; density of the fluid; and other factors. Accordingly, these factors can be manipulated to control the energy level of the excited electrons. Since different tissue structures have different molecular bonds, the present invention can be configured to break the molecular bonds of certain tissue, while having too low an energy to break the molecular bonds of other tissue. For example, fatty tissue, (e.g., adipose) tissue has double bonds that require a substantially higher energy level than 4 to 5 eV to break. Accordingly, the present invention in its current configuration generally does not ablate or remove such fatty tissue. Of course, factors may be changed such that these double bonds can also be broken in a similar fashion as the single bonds (e.g., increasing voltage or changing the electrode configuration to increase the current density at the electrode tips). A more complete description of this phenomena can be found in co-pending U.S. patent application Ser. No. 09/032,375, filed Feb. 27, 1998, the complete disclosure of which is incorporated herein by reference.

The present invention also provides systems, apparatus and methods for selectively removing tumors, e.g., facial tumors, or other undesirable body structures while minimizing the spread of viable cells from the tumor. Conventional techniques for removing such tumors generally result in the production of smoke in the surgical setting, termed an electrosurgical or laser plume, which can spread intact, viable bacterial or viral particles from the tumor or lesion to the surgical team or to other portions of the patient's body. This potential spread of viable cells or particles has resulted in increased concerns over the proliferation of certain debilitating and fatal diseases, such as hepatitis, herpes, HIV and papillomavirus. In the present invention, high frequency voltage is applied between the electrode terminal(s) and one or more return electrode(s) to volumetrically remove at least a portion of the tissue cells in the tumor through the dissociation or disintegration of organic molecules into non-viable atoms and molecules. Specifically, the present invention converts the solid tissue cells into non-condensable gases that are no longer intact or viable, and thus, not capable of spreading viable tumor particles to other portions of the patient's brain or to the surgical staff. The high frequency voltage is preferably selected to effect controlled removal of these tissue cells while minimizing substantial tissue necrosis to surrounding or underlying tissue. A more complete description of this phenomena can be found in co-pending U.S. patent application Ser. No. 09/109,219, filed Jun. 30, 1998, the complete disclosure of which is incorporated herein by reference.

The electrosurgical instrument will comprise a shaft having a proximal end and a distal end which supports one or more electrode terminal(s). The shaft may assume a wide variety of configurations, with the primary purpose being to mechanically support one or more electrode terminal(s) and permit the treating physician to manipulate the electrode(s) from a proximal end of the shaft. Usually, an electrosurgical probe shaft will be a narrow-diameter rod or tube, more usually having dimensions which permit it to be introduced through a cannula into the patient's body. Thus, the probe shaft will typically have a length of at least 5 cm for open procedures and at least 10 cm, more typically being 20 cm, or longer for endoscopic procedures. The probe shaft will typically have a diameter of at least 1 mm and frequently in the range from 1 to 10 mm. For dermatology or other procedures on the skin surface, the shaft will have any suitable length and diameter that would facilitate handling by the surgeon.

The electrosurgical instrument may also be a catheter that is delivered percutaneously and/or endoluminally into the patient by insertion through a conventional or specialized guide catheter, or the invention may include a catheter having an active electrode or electrode array integral with its distal end. The catheter shaft may be rigid or flexible, with flexible shafts optionally being combined with a generally rigid external tube for mechanical support. Flexible shafts may be combined with pull wires, shape memory actuators, and other known mechanisms for effecting selective deflection of the distal end of the shaft to facilitate positioning of the electrode or electrode array. The catheter shaft will usually include a plurality of wires or other conductive elements running axially therethrough to permit connection of the electrode or electrode array and the return electrode to a connector at the proximal end of the catheter shaft. The catheter shaft may include a guide wire for guiding the catheter to the target site, or the catheter may comprise a steerable guide catheter. The catheter may also include a substantially rigid distal end portion to increase the torque control of the distal end portion as the catheter is advanced further into the patient's body. Specific shaft designs will be described in detail in connection with the figures hereinafter.

The active electrode terminal(s) are preferably supported within or by an inorganic insulating support positioned near the distal end of the instrument shaft. The return electrode may be either integrated with the instrument shaft, or another instrument located in close proximity to the distal end of the instrument shaft. The proximal end of the instrument will include the appropriate electrical connections for coupling the return electrode(s) and the electrode terminal(s) to a high frequency power supply, such as an electrosurgical generator. One of more insulators will be positioned around the return electrode to insulate the return electrode from the conductive medium and the patient's body.

The current flow path between the active electrode terminals and the insulator(s) may be generated by submerging the tissue site in an electrical conducting fluid (e.g., within a viscous fluid, such as an electrically conductive gel) or by directing an electrically conducting fluid along a fluid path to the target site (i.e., a liquid, such as isotonic saline, hypotonic saline or a gas, such as argon). The conductive gel may also be delivered to the target site to achieve a slower more controlled delivery rate of conductive fluid. In addition, the viscous nature of the gel may allow the surgeon to more easily contain the gel around the target site (e.g., rather than attempting to contain isotonic saline). A more complete description of an exemplary method of directing electrically conducting fluid between the active and return electrodes is described in U.S. Pat. No. 5,697,281, previously incorporated herein by reference. Alternatively, the body's natural conductive fluids, such as blood, may be sufficient to establish a conductive path between the insulator and the active electrode terminal(s), and to provide the conditions for establishing a vapor layer, as described above. However, conductive fluid that is introduced into the patient is generally preferred over blood because blood will tend to coagulate at certain temperatures. In addition, the patient's blood may not have sufficient electrical conductivity to adequately form a plasma in some applications. Advantageously, a liquid electrically conductive fluid (e.g., isotonic saline) may be used to concurrently “bathe” the target tissue surface to provide an additional means for removing any tissue, and to cool the region of the target tissue ablated in the previous moment.

The power supply may include a fluid interlock for interrupting power to the electrode terminal(s) when there is insufficient conductive fluid around the electrode terminal(s). This ensures that the instrument will not be activated when conductive fluid is not present, minimizing the tissue damage that may otherwise occur. A more complete description of such a fluid interlock can be found in commonly assigned, co-pending U.S. application Ser. No. 09/058,336, filed Apr. 10, 1998, the complete disclosure of which is incorporated herein by reference.

In some procedures, it may also be necessary to retrieve or aspirate the electrically conductive fluid and/or the non-condensible gaseous products of ablation. In addition, it may be desirable to aspirate small pieces of tissue or other body structures that are not completely disintegrated by the high frequency energy, or other fluids at the target site, such as blood, mucus, the gaseous products of ablation, etc. Accordingly, the system of the present invention may include one or more suction lumen(s) in the instrument, or on another instrument, coupled to a suitable vacuum source for aspirating fluids from the target site. In addition, the invention may include one or more aspiration electrode(s) coupled to the distal end of the suction lumen for ablating, or at least reducing the volume of, non-ablated tissue fragments that are aspirated into the lumen. The aspiration electrode(s) function mainly to inhibit clogging of the lumen that may otherwise occur as larger tissue fragments are drawn therein. The aspiration electrode(s) may be different from the ablation electrode terminal(s), or the same electrode(s) may serve both functions. A more complete description of instruments incorporating aspiration electrode(s) can be found in commonly assigned, co-pending patent application entitled “Systems And Methods For Tissue Resection, Ablation And Aspiration”, filed Jan. 21, 1998, the complete disclosure of which is incorporated herein by reference.

As an alternative or in addition to suction, it may be desirable to contain the excess electrically conductive fluid, tissue fragments and/or gaseous products of ablation at or near the target site with a containment apparatus, such as a basket, retractable sheath or the like. This embodiment has the advantage of ensuring that the conductive fluid, tissue fragments or ablation products do not flow through the patient's vasculature or into other portions of the body. In addition, it may be desirable to limit the amount of suction to limit the undesirable effect suction may have on hemostasis of severed blood vessels.

The present invention may use a single active electrode terminal or an array of electrode terminals spaced around the distal surface of a catheter or probe. In the latter embodiment, the electrode array usually includes a plurality of independently current-limited and/or power-controlled electrode terminals to apply electrical energy selectively to the target tissue while limiting the unwanted application of electrical energy to the surrounding tissue and environment resulting from power dissipation into surrounding electrically conductive fluids, such as blood, normal saline, and the like. The electrode terminals may be independently current-limited by isolating the terminals from each other and connecting each terminal to a separate power source that is isolated from the other electrode terminals. Alternatively, the electrode terminals may be connected to each other at either the proximal or distal ends of the catheter to form a single wire that couples to a power source.

In one configuration, each individual electrode terminal in the electrode array is electrically insulated from all other electrode terminals in the array within said instrument and is connected to a power source which is isolated from each of the other electrode terminals in the array or to circuitry which limits or interrupts current flow to the electrode terminal when low resistivity material (e.g., blood, electrically conductive saline irrigant or electrically conductive gel) causes a lower impedance path between the return electrode and the individual electrode terminal. The isolated power sources for each individual electrode terminal may be separate power supply circuits having internal impedance characteristics which limit power to the associated electrode terminal when a low impedance return path is encountered. By way of example, the isolated power source may be a user selectable constant current source. In this embodiment, lower impedance paths will automatically result in lower resistive heating levels since the heating is proportional to the square of the operating current times the impedance. Alternatively, a single power source may be connected to each of the electrode terminals through independently actuatable switches, or by independent current limiting elements, such as inductors, capacitors, resistors and/or combinations thereof. The current limiting elements may be provided in the instrument, connectors, cable, controller or along the conductive path from the controller to the distal tip of the instrument. Alternatively, the resistance and/or capacitance may occur on the surface of the active electrode terminal(s) due to oxide layers which form selected electrode terminals (e.g., titanium or a resistive coating on the surface of metal, such as platinum).

The tip region of the instrument may comprise many independent electrode terminals designed to deliver electrical energy in the vicinity of the tip. The selective application of electrical energy to the conductive fluid is achieved by connecting each individual electrode terminal and the return electrode to a power source having independently controlled or current limited channels. The return electrode(s) may comprise a single tubular member of conductive material proximal to the electrode array at the tip Which also serves as a conduit for the supply of the electrically conducting fluid between the active and return electrodes. Alternatively, the instrument may comprise an array of return electrodes at the distal tip of the instrument (together with the active electrodes) to maintain the electric current at the tip. The application of high frequency voltage between the return electrode(s) and the electrode array results in the generation of high electric field intensities at the distal tips of the electrode terminals with conduction of high frequency current from each individual electrode terminal to the return electrode. The current flow from each individual electrode terminal to the return electrode(s) is controlled by either active or passive means, or a combination thereof, to deliver electrical energy to the surrounding conductive fluid while minimizing energy delivery to surrounding (non-target) tissue.

The application of a high frequency voltage between the return electrode(s) and the electrode terminal(s) for appropriate time intervals effects cutting, removing, ablating, shaping, contracting or otherwise modifying the target tissue. The tissue volume over which energy is dissipated (i.e., a high current density exists) may be more precisely controlled, for example, by the use of a multiplicity of small electrode terminals. In these embodiment, the electrode terminals may have effective diameters or principal dimensions range from about 10 mm to 0.01 mm, preferably from about 2 mm to 0.05 mm, and more preferably from about 1 mm to 0.1 mm. In these embodiments, electrode areas for both circular and non-circular terminals will have a contact area (per electrode terminal) below 50 mm2 for electrode arrays and as large as 75 mm2 for single electrode embodiments. In multiple electrode arrays, the contact area of each electrode terminal is typically in the range from 0.0001 mm2 to 0.5 mm2, and more preferably from 0.001 mm2 to 0.5 mm2. The circumscribed area of the electrode array or electrode terminal is in the range from 0.25 mm2 to 75 mm2, preferably from 0.5 mm2 to 40 mm2. In multiple electrode embodiments, the array will usually include at least two isolated electrode terminals, often at least five electrode terminals, often greater than 10 electrode terminals and even 50 or more electrode terminals, disposed over the distal contact surfaces on the shaft. The use of small diameter electrode terminals increases the electric field intensity and reduces the extent or depth of tissue heating as a consequence of the divergence of current flux lines which emanate from the exposed surface of each electrode terminal.

The area of the tissue treatment surface can vary widely, and the tissue treatment surface can assume a variety of geometries, with particular areas and geometries being selected for specific applications. The geometries can be planar, concave, convex, hemispherical, conical, linear “in-line” array or virtually any other regular or irregular shape. Most commonly, the active electrode(s) or electrode terminal(s) will be formed at the distal tip of the electrosurgical instrument shaft, frequently being planar, disk-shaped, or hemispherical surfaces for use in reshaping procedures or being linear arrays for use in cutting. Alternatively or additionally, the active electrode(s) may be formed on lateral surfaces of the electrosurgical instrument shaft (e.g., in the manner of a spatula), facilitating access to certain body structures in endoscopic procedures.

In some embodiments, the electrode support and the fluid outlet may be recessed from an outer surface of the instrument or handpiece to confine the electrically conductive fluid to the region immediately surrounding the electrode support. In addition, the shaft may be shaped so as to form a cavity around the electrode support and the fluid outlet. This helps to assure that the electrically conductive fluid will remain in contact with the electrode terminal(s) and the return electrode(s) to maintain the conductive path therebetween. In addition, this will help to maintain a vapor layer and subsequent plasma layer between the electrode terminal(s) and the tissue at the treatment site throughout the procedure, which reduces the thermal damage that might otherwise occur if the vapor layer were extinguished due to a lack of conductive fluid. Provision of the electrically conductive fluid around the target site also helps to maintain the tissue temperature at desired levels.

In other embodiments, the active electrodes are spaced from the tissue a sufficient distance to minimize or avoid contact between the tissue and the vapor layer formed around the active electrodes. In these embodiments, contact between the heated electrons in the vapor layer and the tissue is minimized as these electrons travel from the vapor layer back through the conductive fluid to the return electrode. The ions within the plasma, however, will have sufficient energy, under certain conditions such as higher voltage levels, to accelerate beyond the vapor layer to the tissue. Thus, the tissue bonds are dissociated or broken as in previous embodiments, while minimizing the electron flow, and thus the thermal energy, in contact with the tissue.

The electrically conducting fluid should have a threshold conductivity to provide a suitable conductive path between the insulator and the active electrode terminal(s). The electrical conductivity of the fluid (in units of milliSiemans per centimeter or mS/cm) will usually be greater than 0.2 mS/cm, preferably will be greater than 2 mS/cm and more preferably greater than 10 mS/cm. In an exemplary embodiment, the electrically conductive fluid is isotonic saline, which has a conductivity of about 17 mS/cm. Applicant has found that a more conductive fluid, or one with a higher ionic concentration, will usually provide a more aggressive ablation rate. For example, a saline solution with higher levels of sodium chloride than conventional saline (which is on the order of about 0.9% sodium chloride) e.g., on the order of greater than 1% or between about 3% and 20%, may be desirable. Alternatively, the invention may be used with different types of conductive fluids that increase the power of the plasma layer by, for example, increasing the quantity of ions in the plasma, or by providing ions that have higher energy levels than sodium ions. For example, the present invention may be used with elements other than sodium, such as potassium, magnesium, calcium and other metals near the left end of the periodic chart. In addition, other electronegative elements may be used in place of chlorine, such as fluorine.

The voltage difference applied between the return electrode(s) and the electrode terminal(s) will be at high or radio frequency, typically between about 5 kHz and 20 MHz, usually being between about 30 kHz and 2.5 MHz, preferably being between about 50 kHz and 500 kHz, often less than 350 kHz, and often between about 100 kHz and 200 kHz. In some applications, applicant has found that a frequency of about 100 kHz is useful because the tissue impedance is much greater at this frequency. In other applications, such as procedures in or around the heart or head and neck, higher frequencies may be desirable (e.g., 400-600 kHz) to minimize low frequency current flow into the heart or the nerves of the head and neck. The RMS (root mean square) voltage applied will usually be in the range from about 5 volts to 1000 volts, preferably being in the range from about 10 volts to 500 volts, often between about 150 to 350 volts depending on the electrode terminal size, the operating frequency and the operation mode of the particular procedure or desired effect on the tissue (i.e., contraction, coagulation, cutting or ablation). Typically, the peak-to-peak voltage for ablation or cutting with a square wave form will be in the range of 10 to 2000 volts and preferably in the range of 100 to 1800 volts and more preferably in the range of about 300 to 1500 volts, often in the range of about 300 to 800 volts peak to peak (again, depending on the electrode size, the operating frequency and the operation mode). Lower peak-to-peak voltages will be used for tissue coagulation or collagen contraction and will typically be in the range from 50 to 1500, preferably 100 to 1000 and more preferably 120 to 400 volts peak-to-peak (again, these values are computed using a square wave form) Higher peak-to-peak voltages, e.g., greater than about 700 volts peak-to-peak, may be desirable for ablation of harder material, such as bone, depending on other factors, such as the electrode geometries and the composition of the conductive fluid.

As discussed above, the voltage is usually delivered in a series of voltage pulses or alternating current of time varying voltage amplitude with a sufficiently high frequency (e.g., on the order of 5 kHz to 20 MHz) such that the voltage is effectively applied continuously (as compared with e.g., lasers claiming small depths of necrosis, which are generally pulsed about 10 to 20 Hz). In addition, the duty cycle (i.e., cumulative time in any one-second interval that energy is applied) is on the order of about 50% for the present invention, as compared with pulsed lasers which typically have a duty cycle of about 0.0001%.

The preferred power source of the present invention delivers a high frequency current selectable to generate average power levels ranging from several milliwatts to tens of watts per electrode, depending on the volume of target tissue being heated, and/or the maximum allowed temperature selected for the instrument tip. The power source allows the user to select the voltage level according to the specific requirements of a particular neurosurgery procedure, cardiac surgery, arthroscopic surgery, dermatological procedure, ophthalmic procedures, open surgery or other endoscopic surgery procedure. For cardiac procedures and potentially for neurosurgery, the power source may have an additional filter, for filtering leakage voltages at frequencies below 100 kHz, particularly voltages around 60 kHz. Alternatively, a power source having a higher operating frequency, e.g., 300 to 600 kHz may be used in certain procedures in which stray low frequency currents may be problematic. A description of one suitable power source can be found in co-pending patent application Ser. Nos. 09/058,571 and 09/058,336, filed Apr. 10, 1998, the complete disclosure of both applications are incorporated herein by reference for all purposes.

The power source may be current limited or otherwise controlled so that undesired heating of the target tissue or surrounding (non-target) tissue does not occur. In a presently preferred embodiment of the present invention, current limiting inductors are placed in series with each independent electrode terminal, where the inductance of the inductor is in the range of 10 uH to 50,000 uH, depending on the electrical properties of the target tissue, the desired tissue heating rate and the operating frequency. Alternatively, capacitor-inductor (LC) circuit structures may be employed, as described previously in U.S. Pat. No. 5,697,909, the complete disclosure of which is incorporated herein by reference. Additionally, current limiting resistors may be selected. Preferably, these resistors will have a large positive temperature coefficient of resistance so that, as the current level begins to rise for any individual electrode terminal in contact with a low resistance medium (e.g., saline irrigant or blood), the resistance of the current limiting resistor increases significantly, thereby minimizing the power delivery from said electrode terminal into the low resistance medium (e.g., saline irrigant or blood).

It should be clearly understood that the invention is not limited to electrically isolated electrode terminals, or even to a plurality of electrode terminals. For example, the array of active electrode terminals may be connected to a single lead that extends through the catheter shaft to a power source of high frequency current. Alternatively, the instrument may incorporate a single electrode that extends directly through the catheter shaft or is connected to a single lead that extends to the power source. The active electrode(s) may have ball shapes (e.g., for tissue vaporization and desiccation), twizzle shapes (for vaporization and needle-like cutting), spring shapes (for rapid tissue debulking and desiccation), twisted metal shapes, annular or solid tube shapes or the like. Alternatively, the electrode(s) may comprise a plurality of filaments, rigid or flexible brush electrode(s) (for debulking a tumor, such as a fibroid, bladder tumor or a prostate adenoma), side-effect brush electrode(s) on a lateral surface of the shaft, coiled electrode(s) or the like.

In one embodiment, an electrosurgical catheter or probe comprises a single active electrode terminal that extends from an insulating member, e.g., ceramic or silicone, at the distal end of the shaft. The insulating member is preferably a tubular structure that separates the active electrode terminal from a tubular or annular return electrode positioned proximal to the insulating member and the active electrode. In another embodiment, the catheter or probe includes a single active electrode that can be rotated relative to the rest of the catheter body, or the entire catheter may be rotated related to the lead. The single active electrode can be positioned adjacent the abnormal tissue and energized and rotated as appropriate to remove this tissue.

The current flow path between the active electrode terminal(s) and the insulator may be generated by submerging the tissue site in an electrical conducting fluid (e.g., within a viscous fluid, such as an electrically conductive gel) or by directing an electrically conducting fluid along a fluid path to the target site (i.e., a liquid, such as isotonic saline, or a gas, such as argon). This latter method is particularly effective in a dry environment (i.e., the tissue is not submerged in fluid) because the electrically conducting fluid provides a suitable current flow path from the electrode terminal to the return electrode.

Referring to FIG. 1, an exemplary electrosurgical system 5 for treatment of tissue in the body will now be described in detail. Electrosurgical system 5 is generally useful for open procedures, or for minimally invasive procedures, wherein a surgical instrument is introduced through a percutaneous penetration, or through a natural opening in the patient. As shown, electrosurgical system 5 generally includes an electrosurgical probe 20 connected to a power supply 10 for providing high frequency voltage to one or more electrode terminals 42 on probe 20. Probe 20 includes a connector housing 44 at its proximal end, which can be removably connected to a probe receptacle 32 of a probe cable 22. The proximal portion of cable 22 has a connector 34 to couple probe 20 to power supply 10. Power supply 10 has an operator controllable voltage level adjustment 38 to change the applied voltage level, which is observable at a voltage level display 40. Power supply 10 also includes one or more foot pedal(s) 24 and one or more cable(s) 26 which are each removably coupled to receptacle 30 with a cable connector 28. The foot pedal(s) 24 may include a second pedal (not shown) for remotely adjusting the energy level applied to electrode terminals 42, and a third pedal (also not shown) for switching between an ablation mode and a sub-ablation mode (such as coagulation or contraction).

In an exemplary embodiment, a first foot pedal 24 is used to place the power supply into the “ablation” mode and second foot pedal (not shown) places power supply 28 into the “coagulation” mode. The third foot pedal (not shown) allows the user to adjust the voltage level within the “ablation” mode. In the ablation mode, a sufficient voltage is applied to the electrode terminals to establish the requisite conditions for molecular dissociation of the tissue (i.e., forming a plasma with sufficient energy to ablate tissue). As discussed above, the requisite voltage level for ablation will vary depending on the number, size, shape and spacing of the electrodes, the distance in which the electrodes extend from the support member, etc. When the surgeon is using the power supply in the “ablation” mode, voltage level adjustment 40 or third foot pedal may be used to adjust the voltage level to adjust the degree or aggressiveness of the ablation. Of course, it will be recognized that the voltage and modality of the power supply may be controlled by other input devices. However, applicant has found that foot pedals are convenient methods of controlling the power supply while manipulating the probe during a surgical procedure.

In the coagulation mode, the power supply 28 applies a low enough voltage to one or more electrode terminals (or one or more coagulation electrodes) to avoid vaporization of the electrically conductive fluid, formation of a plasma and subsequent molecular dissociation of the tissue. The surgeon may automatically toggle the power supply between the ablation and coagulation modes by alternatively stepping on the appropriate foot pedals. This allows the surgeon to quickly move between coagulation and ablation in situ, without having to remove his/her concentration from the surgical field or without having to request an assistant to switch the power supply. By way of example, as the surgeon is sculpting soft tissue in the ablation mode, the probe typically will simultaneously seal and/or coagulate small severed vessels within the tissue. However, larger vessels, or vessels with high fluid pressures (e.g., arterial vessels) may not be sealed in the ablation mode. Accordingly, the surgeon can simply step on the appropriate foot pedal, automatically lowering the voltage level below the threshold level for ablation, and apply sufficient pressure onto the severed vessel for a sufficient period of time to seal and/or coagulate the vessel. After this is completed, the surgeon may quickly move back into the ablation mode by stepping on the appropriate foot pedal A specific design of a suitable power supply for use with the present invention can be found in U.S. patent application Ser. No. 09/058,571, filed Apr. 10, 1998, previously incorporated herein by reference.

Referring now to FIG. 2, a representative high frequency power supply for use according to the principles of the present invention will now be described. The high frequency power supply of the present invention is configured to apply a high frequency voltage of about 10 to 1000 volts RMS between one or more electrode terminals (and/or coagulation electrode) and one or more return electrodes. In the exemplary embodiment, the power supply applies about 70-350 volts RMS in the ablation mode and about 20 to 90 volts in a subablation mode, preferably 45 to 70 volts in coagulation mode (these values will, of course, vary depending on the probe configuration attached to the power supply and the desired mode of operation).

The preferred power source of the present invention delivers a high frequency current selectable to generate average power levels ranging from several milliwatts to tens of watts per electrode, depending on the volume of target tissue being heated, and/or the maximum allowed temperature selected for the probe tip. The power source allows the user to select the voltage level according to the specific requirements of a particular procedure, e.g., arthroscopic surgery, dermatological procedure, interventional cardiology procedures, open surgery or other endoscopic surgery procedure.

As shown in FIG. 2, the power supply generally comprises a radio frequency (RF) power oscillator 100 having output connections for coupling via a power output signal 102 to the load impedance, which is represented by the electrode assembly when the electrosurgical probe is in use. In the representative embodiment, the RF oscillator operates at about 100 kHz. The RF oscillator is not limited to this frequency and may operate at frequencies of about 300 kHz to 600 kHz. In particular, for cardiac applications, the RF oscillator will preferably operate in the range of about 400 kHz to about 600 kHz. The RF oscillator will generally supply a square wave signal with a crest factor of about 1 to 2. Of course, this signal may be a sine wave signal or other suitable wave signal depending on the application and other factors, such as the voltage applied, the number and geometry of the electrodes, etc. The power output signal 102 is designed to incur minimal voltage decrease (i.e., sag) under load. This improves the applied voltage to the electrode terminals and the return electrode, which improves the rate of volumetric removal (ablation) of tissue.

Power is supplied to the oscillator 100 by a switching power supply 104 coupled between the power line and the RF oscillator rather than a conventional transformer. The switching power supply 140 allows the generator to achieve high peak power output without the large size and weight of a bulky transformer. The architecture of the switching power supply also has been designed to reduce electromagnetic noise such that U.S. and foreign EMI requirements are met. This architecture comprises a zero voltage switching or crossing, which causes the transistors to turn ON and OFF when the voltage is zero. Therefore, the electromagnetic noise produced by the transistors switching is vastly reduced. In an exemplary embodiment, the switching power supply 104 operates at about 100 kHz.

A controller 106 coupled to the operator controls 105 (i.e., foot pedals and voltage selector) and display 116, is connected to a control input of the switching power supply 104 for adjusting the generator output power by supply voltage variation. The controller 106 may be a microprocessor or an integrated circuit. The power supply may also include one or more current sensors 112 for detecting the output current. The power supply is preferably housed within a metal casing which provides a durable enclosure for the electrical components therein. In addition, the metal casing reduces the electromagnetic noise generated within the power supply because the grounded metal casing functions as a “Faraday shield”, thereby shielding the environment from internal sources of electromagnetic noise.

The power supply generally comprises a main or mother board containing generic electrical components required for many different surgical procedures (e.g., arthroscopy, urology, general surgery, dermatology, neurosurgery, etc.), and a daughter board containing application specific current-limiting circuitry (e.g., inductors, resistors, capacitors and the like). The daughter board is coupled to the mother board by a detachable multi-pin connector to allow convenient conversion of the power supply to, e.g., applications requiring a different current limiting circuit design. For arthroscopy, for example, the daughter board preferably comprises a plurality of inductors of about 200 to 400 microhenries, usually about 300 microhenries, for each of the channels supplying current to the electrode terminals.

Alternatively, in one embodiment, current limiting inductors are placed in series with each independent electrode terminal, where the inductance of the inductor is in the range of 10 uH to 50,000 uH, depending on the electrical properties of the target tissue, the desired tissue heating rate and the operating frequency. Alternatively, capacitor-inductor (LC) circuit structures may be employed, as described previously in co-pending PCT application No. PCT/US94/05168 (now U.S. Pat. No. 5,697,909), the complete disclosure of which is incorporated herein by reference. Additionally, current limiting resistors may be selected. Preferably, these resistors will have a large positive temperature coefficient of resistance so that, as the current level begins to rise for any individual electrode terminal in contact with a low resistance medium (e.g., saline irrigant or conductive gel), the resistance of the current limiting resistor increases significantly, thereby minimizing the power delivery from said electrode terminal into the low resistance medium (e.g., saline irrigant or conductive gel).

Power output signal may also be coupled to a plurality of current limiting elements (not shown), which are preferably located on the daughter board since the current limiting elements may vary depending on the application. In this embodiment, a high frequency power supply is connected to a multiplicity of current limiting elements, typically being inductors having an inductance in the range of about 100 to 5000 microhenries, with the particular value depending on the electrode terminal dimensions, the desired ablation rates, and the like. Capacitors having capacitance values in the range of about 200 to 10,000 picofarads may also be used as the current limiting elements. It would also be possible to use resistors as current limiting elements. The current limiting elements any also be part of a resonant circuit structure, as described in detail in PCT/US94/05168, previously incorporated herein by reference.

FIGS. 3 and 4 illustrate an exemplary electrosurgical probe 20 constructed according to the principles of the present invention. As shown in FIG. 3, probe 20 generally includes an elongated shaft 100 which may be flexible or rigid, a handle 204 coupled to the proximal end of shaft 100 and an electrode support member 102 coupled to the distal end of shaft 100. Shaft 100 preferably comprises an insulating material, such as plastic, polyimide or the like. In this embodiment, the return electrode 112 will have a connector extending through the shaft to couple the return electrode 112 with high frequency power supply 28. Alternatively, shaft 100 may comprise an electrically conducting material, usually a metal, such as tungsten, stainless steel alloys, platinum or its alloys, titanium or its alloys, molybdenum or its alloys, and nickel or its alloys. In this embodiment, the return electrode 112 may comprise the shaft itself, or a separate electrode insulated from the shaft. In both of these embodiments, shaft 100 will also include an electrically insulating jacket 108 (see FIG. 4), which is typically formed as one or more electrically insulating sheaths or coatings, such as polytetrafluoroethylene, polyimide, and the like. The provision of the electrically insulating jacket over the shaft prevents direct electrical contact between these metal elements and any adjacent body structure or the surgeon. Such direct electrical contact between a body structure (e.g., tendon) and an exposed electrode could result in unwanted heating and necrosis of the structure at the point of contact causing necrosis.

Handle 204 typically comprises a plastic material that is easily molded into a suitable shape for handling by the surgeon. Handle 204 defines an inner cavity (not shown) that houses the electrical connections, and provides a suitable interface for connection to an electrical connecting cable 22 (see FIG. 1). The probe will typically include a coding resistor (not shown) having a value selected to program different output ranges and modes of operation for the power supply. This allows a single power supply to be used with a variety of different probes in different applications (e.g., dermatology, cardiac surgery, neurosurgery, arthroscopy, etc).

Referring to FIG. 4, electrode support member 102 extends from the distal end of shaft 100 (usually about 1 to 20 mm), and provides support for a plurality of electrically isolated electrode terminals 120. Electrode support member 102 and electrode terminals 120 are preferably secured to a tubular support member 122 within shaft 100 by adhesive (not shown). The electrode terminals 120 may be constructed using round, square, rectangular or other shaped conductive metals. By way of example, the electrode terminal materials may be selected from the group including stainless steel, tungsten and its alloys, molybdenum and its alloys, titanium and its alloys, nickel-based alloys, as well as platinum and its alloys. Electrode support member 102 is preferably an inorganic material, such as a silicone, ceramic, glass or glass/ceramic composition (e.g., aluminum oxide, titanium nitride or the like). Alternatively, electrode support member 102 may include the use of high-temperature biocompatible plastics such as polyether-ether-keytone (PEEK) manufactured by Vitrex International Products, Inc. or polysulfone manufactured by GE Plastics. The adhesive may, by way of example, be an epoxy (e.g., Master Bond EP42HT manufactured by Master Bond) or a silicone-based adhesive.

As discussed above, cannula 122 is covered with an electrically insulating sleeve 108, which may be a coating (e.g., nylon) or heat shrinkable plastic (e.g., fluropolymer or polyester). The material of sleeve will depend on the desired characteristics of the capacitor, i.e., dielectric constant, thickness, etc. As shown in FIG. 4, a return electrode 112 is located on the outer surface of cannula 122 radially inward from sleeve 108. In this manner, return electrode 112 is electrically insulated from the active electrode terminals 120 by sleeve 108. Return electrode 112 preferably comprises a single annular piece surrounding cannula 122, but it may also comprises a plurality of small electrodes circumferentially spaced around cannula 122. The length of the return electrode 112, is preferably in the range from 1 mm to 30 mm and more preferably in the range from 2 mm to 20 mm. The spacing between the most distal portion of the return electrode 112 and the plane of the tissue treatment surface of the electrode support member 101, is preferably in the range from 0.5 mm to 30 mm. The thickness of the electrically insulating sleeve 108 is preferably in the range from 0.01 mm to 0.5 mm and more preferably in the range from 0.02 mm to 0.2 mm.

In the embodiment shown in FIGS. 3 and 4, the electrically conducting fluid is delivered from a fluid delivery element (not shown) that is separate from probe 20. In arthroscopic surgery, for example, the body cavity will be flooded with isotonic saline and the probe 20 will be introduced into this flooded cavity. Electrically conducting fluid will be continually resupplied to maintain the conduction path between return electrode 112 and electrode terminals 102. The electrically conductive fluid creates a conductive path between active electrode terminals 120 and the portion of insulator 108 around return electrode 112, as shown by current flux lines 130.

FIG. 5 illustrates another embodiment of the present invention incorporating an aspiration lumen and an aspiration electrode designed to ablate tissue fragments as they as aspirated into the lumen. As shown, an electrosurgical probe 300 includes an elongated shaft 304 which may be flexible or rigid, a handle 306 coupled to the proximal end of shaft 304 and an electrode support member 308 coupled to the distal end of shaft 304. Probe 300 further includes an active screen electrode 302 and a return electrode 310 spaced proximally from active screen electrode 302. Similar to previous embodiments, return electrode 310 is insulated from active screen electrode 302 by an outer insulating jacket 309 having a thickness in the range of about 0.01 to 0.5 mm. Probe 300 is designed to operate in an electrically conductive medium (e.g., saline) to provide a conductive path between the portion of jacket 309 surrounding return electrode 310 and the active screen 302.

In this embodiment, active screen electrode 302 and support member 308 are configured such that the active electrode 302 is positioned on a lateral side of the shaft 304 (e.g., 90 degrees from the shaft axis) to allow the physician to access tissue that is offset from the axis of the portal or arthroscopic opening into the joint cavity in which the shaft 304 passes during the procedure. To accomplish this, probe 300 includes an electrically insulating cap 320 coupled to the distal end of shaft 304 and having a lateral opening 322 for receiving support member 308 and screen electrode 302.

The probe 300 further includes a suction connection tube 314 for coupling to a source of vacuum, and an inner suction lumen 312 for aspirating excess fluids, tissue fragments, and/or products of ablation (e.g., bubbles) from the target site. In addition, suction lumen 312 allows the surgeon to draw loose tissue, e.g., synovial tissue, towards the screen electrode 302, as discussed above. Typically, the vacuum source is a standard hospital pump that provides suction pressure to connection tube 314 and lumen 312. However, a pump may also be incorporated into the high frequency power supply. A more complete description of a suitable vacuum system and alternative embodiments for aspiration electrodes for an electrosurgical probe can be found in U.S. patent application Ser. No. 09/197,013, the complete disclosure of which in incorporated herein by reference.

The screen electrode 302 will comprise a conductive material, such as tungsten, titanium, molybdenum, stainless steel, aluminum, gold, copper or the like. In some embodiments, it may be advantageous to construct the active and return electrodes of the same material to eliminate the possibility of DC currents being created by dissimilar metal electrodes. Screen electrode 302 will usually have a diameter in the range of about 0.5 to 8 mm, preferably about 1 to 4 mm, and a thickness of about 0.05 to about 2.5 mm, preferably about 0.1 to 1 mm. Electrode 302 will comprise a plurality of holes 324 having sizes that may vary depending on the particular application and the number of holes (usually from one to 50 holes, and preferably about 3 to 20 holes). Holes 324 will typically be large enough to allow ablated tissue fragments to pass through into suction lumen 312, typically being about 2 to 30 mils in diameter, preferably about 5 to 20 mils in diameter. In some applications, it may be desirable to only aspirate fluid and the gaseous products of ablation (e.g., bubbles) so that the holes may be much smaller, e.g., on the order of less than 10 mils, often less than 5 mils.

Referring now to FIG. 6, an exemplary electrosurgical probe 410 for treatment of tissue in ‘dry fields’ will now be described in detail. Of course, probe 410 may also be used in ‘wet field’, i.e., the target site is immersed in electrically conductive fluid. However, this probe is particularly useful in ‘dry fields’ where the fluid is preferably delivered through electrosurgical probe to the target site. As shown, the probe 410 includes a shaft 100, a proximal handle 404, a distal support member 468 and an electrode assembly including a return electrode 462 proximally spaced from one or more electrode terminals 464. Similar to previous embodiments, the return electrode 462 is insulated from electrode terminals 464 by an insulator (not shown). To complete the current path between electrode terminals 464 and the insulator, electrically conducting fluid (e.g., isotonic saline) is caused to flow therebetween. In the representative embodiment, probe 410 includes a fluid connector 435 for coupling a fluid tube 433 to a source of electrically conductive fluid, such as a pump or a gravity driven fluid source. The electrically conducting fluid is delivered through fluid tube 433 to opening 437, as described above.

In the representative embodiment, fluid tube 433 comprises peek tubing or a similar type of tubing material. In alternative embodiments, the fluid path may be formed in probe 410 by, for example, an inner lumen or an annular gap between the return electrode 462 and a tubular support member within shaft 100. This annular gap may be formed near the perimeter of the shaft 100 such that the electrically conducting fluid tends to flow radially inward towards the target site, or it may be formed towards the center of shaft 100 so that the fluid flows radially outward. In both of these embodiments, a fluid source (e.g., a bag of fluid elevated above the surgical site or suitable pumping device), is coupled to probe 410 via a fluid supply tube (not shown) that may or may not have a controllable valve.

Referring to FIGS. 7 and 8, the electrosurgical device according to the present invention may also be configured as an elongate catheter system 600 including portions with sufficient flexibility to permit introduction into the body and to the target site through one or more vascular lumen(s). As shown, catheter system 600 generally comprises an electrosurgical catheter 660 connected to a power supply 628 by an interconnecting cable 686 for providing high frequency voltage to a target tissue site and an irrigant reservoir or source 600 for providing electrically conducting fluid to the target site. Catheter 660 generally comprises an elongate, flexible shaft body 662 including a tissue removing or ablating region 664 at the distal end of body 662. The proximal portion of catheter 660 includes a multi-lumen fitment 614 which provides for interconnections between lumens and electrical leads within catheter 660 and conduits and cables proximal to fitment 614. By way of example, a catheter electrical connector 696 is removably connected to a distal cable connector 694 which, in turn, is removably connectable to generator 628 through connector 692. One or more electrically conducting lead wires (not shown) within catheter 660 extend between one or more active electrodes 663 at tissue ablating region 664 and one or more corresponding electrical terminals (also not shown) in catheter connector 696 via active electrode cable branch 687. Similarly, one or more return electrodes 666 at tissue ablating region 664 are coupled to a return electrode cable branch 689 of catheter connector 696 by lead wires (not shown). Of course, a single cable branch (not shown) may be used for both active and return electrodes.

Catheter body 662 may include reinforcing fibers or braids (not shown) in the walls of at least the distal ablation region 664 of body 662 to provide responsive torque control for rotation of electrode terminals during tissue engagement. This rigid portion of the catheter body 662 preferably extends only about 7 to 10 mm while the remainder of the catheter body 662 is flexible to provide good trackability during advancement and positioning of the electrodes adjacent target tissue.

As shown in FIG. 8, conductive fluid 630 is provided to tissue ablation region 664 catheter 660 via a lumen (690) within catheter 660. Fluid is supplied to lumen from the source along a conductive fluid supply line 602 and a conduit 603, which is coupled to the inner catheter lumen at multi-lumen fitment 614. The source of conductive fluid (e.g., isotonic saline) may be an irrigant pump system (not shown) or a gravity-driven supply, such as an irrigant reservoir 600 positioned several feet above the level of the patient. A control valve 604 may be positioned at the interface of fluid supply line 602 and conduit 603 to allow manual control of the flow rate of electrically conductive fluid 630. Alternatively, a metering pump or flow regulator may be used to precisely control the flow rate of the conductive fluid.

System 600 further includes an aspiration or vacuum system (not shown) to aspirate liquids and gases from the target site. The aspiration system will usually comprise a source of vacuum coupled to fitment 614 by a aspiration connector 605.

Referring to FIG. 8, the working end 664 of catheter 660 comprises an active electrode 692, an inorganic support member 694, a return electrode 696 and an insulator 698. As shown, active electrode 692 preferably comprises a single, tubular shaped electrode extending about 0.5 to 10 mm from support member 694. Return electrode 696 comprises an annular electrode coupled to a tubular cannula 700 that makes up the shaft of catheter 660. Insulator 698 comprises an electrically insulating jacket wrapped around cannula 700, similar to previous embodiments.

Applicant has found that increasing the current densities around the electrode terminal(s) can lead to higher energy levels in the ionized plasma. This, in turn, allows the ionized plasma to break stronger molecular bonds, such as those present in bone or calcified fragments. Since the electrically conductive fluid between the target site and electrode terminal(s) is transformed into an ionized vapor layer or plasma, the number of charged particles which can be accelerated against the target also determines the removal rate. In addition, the conductivity of the fluid may have an effect on the strength of the plasma field created at the end of the probe. Typically, isotonic saline with 0.9% concentration of sodium chloride is used with the probe. By increasing the sodium chloride concentration to greater than 0.9% and preferably between about 3% and 20%, the increased concentration provides for improved tissue ablation rates. This concept of using a hypertonic saline with enhanced conductivity and increased numbers of charged particles is of particular use in bone removal processes or in other procedures requiring aggressive volumetric removal.

Applicant has also found that the plasma layer typically requires a higher voltage level to initiate a plasma than to sustain the plasma once it has been initiated. In addition, it has been found that some conductive solutions facilitate the initiation of the plasma layer, rather than the energy level of the plasma, as discussed above. For example, it has been found that saline solutions having concentrations less than isotonic saline (i.e., less than 0.9% sodium chloride) facilitate the initiation of the plasma layer. This may be useful in applications where initiation of the plasma layer is more difficult, such as applications where a suction pressure is applied near the electrode terminal(s). A more complete description of this type of application, and the devices that carry out simultaneous suction and ablation can be found in U.S. patent application Ser. No. 09/010,382, filed Jan. 21, 1998, the complete disclosure of which is incorporated herein by reference for all purposes.

In another embodiment, the active electrodes are spaced from the tissue a sufficient distance to minimize or avoid contact between the tissue and the vapor layer formed around the active electrodes. In these embodiments, contact between the heated electrons in the vapor layer and the tissue is minimized as these electrons travel from the vapor layer back through the conductive fluid to the return electrode. The ions within the plasma, however, will have sufficient energy, under certain conditions such as higher voltage levels, to accelerate beyond the vapor layer to the tissue. Thus, the tissue bonds are dissociated or broken as in previous embodiments, while minimizing the electron flow, and thus the thermal energy, in contact with the tissue. 

What is claimed is:
 1. A method for applying electrical energy to a body structure comprising: positioning a first electrode adjacent to or in contact with a body structure; positioning a second electrode in the region of the body structure; electrically insulating the first and second electrodes from each other; applying a sufficient high frequency voltage difference between the first and second electrodes to modify at least a portion of the body structure.
 2. The method of claim 1 further comprising applying a sufficient high frequency voltage difference between the first and second electrodes to ablate at least a portion of the body structure.
 3. The method of claim 3 further comprising submersing the first electrode in electrically conductive fluid.
 4. The method of claim 3 further comprising positioning an insulator between the first and second electrodes and contacting the insulator with the electrically conductive fluid.
 5. The method of claim 1 further comprising vaporizing a portion of the electrically conductive fluid adjacent to the active electrode.
 6. A method for applying electrical energy to a body structure comprising: positioning a capacitor in the region of a body structure; and applying a sufficient potential difference across the capacitor to modify at least a portion of the body structure.
 7. The method of claim 6 further comprising applying a sufficient potential difference across the capacitor to ablate at least a portion of the body structure.
 8. The method of claim 6 wherein the capacitor comprises first and second conductors and an insulator therebetween, the method further comprising delivering electrically conductive fluid between one of the conductors and the insulator.
 9. The method of claim 8 wherein the conductors are spaced from each other by a distance of about 1 to 50 mm.
 10. A system for applying electrical energy to a body structure at a target site comprising: an electrosurgical instrument having a shaft with a proximal end, a distal end and an electrode assembly at the distal end of the shaft; and wherein the electrode assembly comprises a first electrode having an exposed surface for applying electrical energy to the body structure and a second electrode spaced from the first electrode and electrically insulated from the first electrode.
 11. The system of claim 10 wherein the first and second electrodes are spaced by a distance of about 1 to 50 mm.
 12. The system of claim 10 wherein the first electrode is located at the distal end of the shaft, and the second electrode is spaced proximally from said first electrode.
 13. The system of claim 10 wherein the second electrode is surrounded by an electrically insulating jacket having a thickness of about 0.01 to 0.5 mm.
 14. The system of claim 10 further comprising a fluid delivery element for delivering electrically conductive fluid between the first and second electrodes.
 15. A system for applying electrical energy to a body structure at a target site comprising: an instrument having a shaft with a proximal end portion and a distal end portion; a capacitor coupled to the distal end portion of the shaft; and a connector for coupling the capacitor to a high frequency power supply.
 16. The system of claim 15 wherein the capacitor comprises first and second conductors and an insulator therebetween.
 17. The system of claim 15 wherein the capacitor is configured, upon the application of a sufficient high frequency potential difference, to allow a sufficient flow of current across the capacitor to ablate at least a portion of the body structure. 